Waveguides can be used in displays, in a variety of sizes, for example for televisions to near-eye displays (NED)s. A typical example of a near-eye display is a head mounted display (HMD).
In an exemplary waveguide display, for example one as exemplified by Upatnieks U.S. Pat. No. 4,711,512, light is coupled into a waveguide mode by an input diffractive element and coupled out to the eye by a second diffractive element. The practical field of view in which data can be displayed by a waveguide display is around thirty (30) degrees due to the range of angles that can be supported by the waveguide. For example, crown glass such as N-BK7 has a refractive index of approximately 1.52 which makes the critical angle for supporting total internal reflection approximately 42.2° in glass. At the other end of the range, light could go parallel to the waveguide surface but this would not be practical since the light would not intercept the second output diffractive element to enable out-coupling of the light. Furthermore, the period between reflections has to be managed such that no gaps can appear in the exit pupil (a balance between parameters such as entrance pupil size, angle of reflection and thickness of the waveguide. There is therefore a practical limit for the angle range beyond a critical angle.
One way of increasing the field of view (FOV) is to have multiple waveguide layers, each covering a different angular portion. If Bragg Gratings are used to couple light into and out of the waveguide, they typically have angular bandwidth limitations lower than what can be supported in the waveguide. For example, this could be limited to 10 degrees angular bandwidth In this spatial approach, the number of layers can start making the waveguide too thick for a consumer product. Another approach for expanding the field of view has been to use multiple switchable gratings for time sequentially generating the field of view fast enough so a user does not notice flicker. In a typical HMD geometry, whether using fixed or switchable gratings in either a spatial or time layering of the FOV, the angular bandwidth provided by each can go down to less than 10 degrees in the tangential direction.